Round of a half cut cylinder with subsurf
Add a cylinder;
Apply E and S to the bases and delete created faces. Remove doubles;
Join the edges with Bridge Edge Loops to make inner faces. The result should look as below;

In face selection mode select, with B, and delete half of the cylinder created thus;
Make faces where the crossection was made (see the image below);



Done.

If you want you may make more quads with Ctrl+R